Ugh. This is the first I've heard of it and it really bothers me. I have dietary issues - I have to be very careful about sodium content in my food and also follow a migraine diet which means avoiding processed, overly-seasoned food. I also like to snack frequently
All of this makes dining at Disney a challenge at the best of times.
As a result, I often take snacks (an apple, a juice box) and definitely a bottle of water so that I don't have to rely on eating Disney food 100 % of the time, thereby lessening the chance that I'll get sick. Bear in mind, when I go it's usually for a week, and we stay on-property, so we're really relying on Disney food for our main meals. It's nice to have control over what I'm eating some of the time.
I've had many, many trips to Disney and I've never seen anything really egregious in terms of people bringing their own food into the park. For the most part, what I see is a mom handing out baggies of carrots to her kids, or the much-mentioned goldfish crackers.
This goes beyond questions of money - it's also a question of convenience. If you have three kids who are about to go into meltdown because they need a snack, it's not always practical to line up and buy something. At busy times, the line-ups for quick-service can be really long. I don't blame parents for packing snacks for their kids, partly to save money but just as importantly for the sake of time and convenience. And I don't think I'm doing anything wrong by taking snacks that I know fit my dietary needs.
If Disney actually enforces this policy, they're really going backwards in terms of offering the best experience to their customers. The overall experience is not just about what Disney offers (attractions, dining, etc...) but also about what flexibility guests can have. This is especially true if you're there for more than a one-day trip.